1. Field of the Invention
The present invention relates to a mixture for powder metallurgy product and a method for producing the mixture. Further, the present invention relates to a powder metallurgy product and a method for producing the powder metallurgy product.
2. Description of the Background
In powder metallurgy, after some kinds of powders are mixed at a predetermined ratio, the mixed powder is formed into a desired shape under pressure with die sets and then sintered to be a final metallurgy product.
One of the advantages of powder metallurgy products is that machining operation is unnecessary because powder metallurgy products having a substantially final shape may be formed in dies without machining operation. Recently, higher precision and more complex shapes have been required. Accordingly, machining operations have been required even for powder metallurgy products. However, generally, powder metallurgy products have poor machinability.
U.S. Pat. 5,938,814 and Japanese Examined Patent Publication (kokoku) 56-45964 (hereinafter referred to as the xe2x80x9c""964 publicationxe2x80x9d) disclose steel powder having good machinability. The contents of these references are incorporated herein by reference in their entirety.
In the steel powder disclosed in the ""964 publication, the steel powder contains S of 0.15 to 0.5 weight percent (wt %) and Mn of at most an amount greater than a Mn/S balance amount by 0.3 weight percent. Mn is used for combining with S. MnS is not easily oxidized after Mn combines with S.
Generally, powder metallurgy products have inferior mechanical strength. The reason is presumed that powder metallurgy products have many pores therein, because powder metallurgy products are produced by being formed under pressure and being sintered.
An object of the present invention is to provide a powder metallurgy product which has improved machinability without substantially deteriorating fatigue strength.
According to one aspect of the invention, a mixture for a powder metallurgy product includes iron powder, graphite powder and copper (Cu) of about 3.0 to about 5.0 weight percent. Iron powder includes iron grains which contain MnS therein. The mixture contains the MnS of about 0.65 to about 1.40 weight percent. The graphite powder is contained in the mixture such that an amount of carbon (C) in the powder metallurgy product is about 0.3 to about 0.7 weight percent. An amount (wt % C) of the carbon and an amount (wt % Cu) of the copper is determined to obtain a target fatigue strength FS (MPa) and a target hardness HR (HRB) based on a relation
FS=66.63xc3x97(wt % C)+22.61xc3x97(wt % Cu)+280.84
HR=22.96xc3x97(wt % C)+2.99xc3x97(wt % Cu)+78.91.
According to another aspect of the invention, a powder metallurgy product which is made from a mixture which includes iron powder, graphite powder and copper (Cu) of about 3.0 to about 5.0 weight percent. Iron powder includes iron grains which contain MnS therein. The mixture contains the MnS of about 0.65 to about 1.40 weight percent. The graphite powder is contained in the mixture such that an amount of carbon (C) in the powder metallurgy product is about 0.3 to about 0.7 weight percent. An amount (wt % C) of the carbon and an amount (wt % Cu) of the copper is determined to obtain a target fatigue strength FS (MPa) and a target hardness HR (HRB) based on a relation
FS=66.63xc3x97(wt % C)+22.61xc3x97(wt % Cu)+280.84
HR=22.96xc3x97(wt % C)+2.99xc3x97(wt % Cu)+78.91.
According to yet another aspect of the invention, a method for producing a mixture for a powder metallurgy product includes depositing MnS in iron grains in iron powder; adding graphite powder to the iron powder such that an amount of carbon (C) in the powder metallurgy product is about 0.3 to about 0.7 weight percent; adding to the iron powder copper (Cu) of about 3.0 to about 5.0 weight percent; and determining an amount (wt % C) of the carbon and an amount (wt % Cu) of the copper to obtain a target fatigue strength FS (MPa) and a target hardness HR (HRB) based on a relation
FS=66.63xc3x97(wt % C)+22.61xc3x97(wt % Cu)+280.84
HR=22.96xc3x97(wt % C)+2.99xc3x97(wt % Cu)+78.91.
The mixture contains the MnS of about 0.65 to about 1.40 weight percent.
According to yet another aspect of the invention, a method for producing a powder metallurgy product includes producing a mixture during a mixture producing process; forming the mixture to a green compact under pressure; and sintering the green compact. The mixture producing process includes depositing MnS in iron grains in iron powder; adding graphite powder to the iron powder such that an amount of carbon (C) in the powder metallurgy product is about 0.3 to about 0.7 weight percent; adding to the iron powder copper (Cu) of about 3.0 to about 5.0 weight percent; and determining an amount (wt % C) of the carbon and an amount (wt % Cu) of the copper to obtain a target fatigue strength FS (MPa) and a target hardness HR (HRB) based on a relation
FS=66.63xc3x97(wt % C)+22.61xc3x97(wt % Cu)+280.84
HR=22.96xc3x97(wt % C)+2.99xc3x97(wt % Cu)+78.91.
The mixture contains the MnS of about 0.65 to about 1.40 weight percent.
According to the other aspect of the invention, a powder metallurgy product includes iron, carbon (C) of about 0.3 to about 0.7 weight percent, and copper (Cu) of about 3.0 to about 5.0 weight percent. The iron includes iron grains which contain MnS therein. The product containing the MnS of about 0.65 to about 1.40 weight percent. An amount (wt % C) of the carbon and an amount (wt % Cu) of the copper is determined to obtain a target fatigue strength FS (MPa) and a target hardness HR (HRB) based on a relation
FS=66.63xc3x97(wt % C)+22.61xc3x97(wt % Cu)+280.84
HR=22.96xc3x97(wt % C)+2.99xc3x97(wt % Cu)+78.91.